Actually he IS in charge the Deathknights.
Every new Deathknight Swears allegiance to HIM specifically.
The Deathlord is HIS champion. They Do no lead the Ebon Blade, they run it FOR him, everything of note you do is merely acting as a vessel for his power to flow through. They are the Arthas to his Ner’zhul.
However, Bolvar has made ZERO efforts that we know of to increase his powers. (We know the Ebon Blade was cooporating with him during WoD, hunting the Cult of the Damned in Draenor as well as looking for information on Ner’zhul that might give them more insight into how to free Bolvar from eventually becoming a Corrupt Lich King as well) He has what he inherited basically, plus however much strength he gained from making new Death Knights (assuming he kept piece of their souls), and whatever percentage of soul power went to him via the Death Knights and Deathlord’s actions, if any.
Where as Ner’zhul and Arthas both went ham on stealing souls to bolster their powers, since their strength is directly proportional to the amount of souls they’ve stolen. (something Sylvanas also apparently is able to do, through her pact with the Jailer, similar to when Ner’zhul sent Arthas a bunch of extra power to help him fulfill his tasks, albeit she kept that fact hidden about the true extent of what she can do now).
We’ve seen no indications of Bolvar EVER doing that. He’s simply been a minder of the dead. Aside for Crafting his Hammer at some point between Legion and SL (probably made by the Ebon Blade for him after officially rejoining his service during Legion (similar to how they made Shadowmourne for us)).